This transgene contains the coding sequence for the Cre-ERT2 fusion protein, followed by a polyadenylation signal, inserted into a mouse genomic artificial chromosome (BAC) at the ATG transcription initiation codon of the solute carrier family 6 (neurotransmitter transporter, serotonin), member 4 gene so that expression of the tamoxifen-activated recombinase is driven by the regulatory sequences of the mouse gene. ERT2 is a triply mutated (G400V, M543A, L544A) human estrogen receptor ligand-binding domain that renders the fusion protein highly sensitive to activation by 4-hydroxytamoxifen, but unresponsive to the receptor's native ligands.